Siapa yang bersama kamu? Answer: Whom are you with?
I think this is the wrong way around.
(1) Siapa yang bersama kamu? = Who is with you?
(2) Kamu bersama siapa? = Who are you with?
I think Duolingo being a grammar nazi to this extent detracts from the language learning bit. 'Who' may be technically incorrect but practically speaking everyone except for English teachers or maybe Phd's will ask 'who are you with' in everyday conversation and should be acceptable as a slang interpretation here.
Is 'Siapa yang bersama kamu?' a formal sentence?
Whether 'whom' is required or not in the English sentence should be irrelevant as it has been falling out of use for quite some time now, except in formal (written) English.
'Together' is the word that doesn't belong in the answer given. Speakers of other Asian languages (e.g. Japanese) make this same mistake when translating to English.
However, even Google translate can figure it out nowadays: Indonesian to English: Who is with you? Malay to English: Who's with you?
If 'yang' is what is being taught in L2 (the target language) here, then they need to figure out a way to do it that makes sense in the L1 (first language).
Perhaps it should be something like, 'Who is THAT (boy/girl/person) with you?' I have no idea. In any case, the Duolingo Indonesian to English translations still need a lot of work.
Much of this section should not get out of beta. This one is especially bad.
Hmm. I wrote this for an answer a couple of questions ago, and it was marked wrong, for the same English phrase. WHOM are you with is correct. The subject of the sentence is YOU. Whom is the object here.